1. Field of the Invention
The present invention relates to a system for diagnosing an engine exhaust gas purifying device using a catalytic converter and an air/fuel ratio or oxygen concentration (hereinafter also referred to as air/fuel ratio) sensor.
2. Description of the Related Art
A device for purifying an exhaust gas from an engine mainly comprises a catalytic converter and an air/fuel ratio feedback (closed-loop) control unit. The catalytic converter is disposed in an exhaust manifold for eliminating HC, NOx and CO from the exhaust gas. It is necessary to keep the air/fuel ratio constant in order to sufficiently perform the function of the catalytic converter. Accordingly, an oxygen sensor is disposed upstream of the catalytic converter so that the air/fuel ratio feedback control unit controls the fuel supply quantity to provide a desired air/fuel ratio.
When the performance of the oxygen sensor provided upstream of the catalytic converter deteriorates in a usual three-way catalyst of the above mentioned prior art, the air/fuel ratio does not fall into a narrow range around the stoichiometric air/fuel ratio, resulting in a reduction in conversion efficiency of harmful components. When the performance of the catalytic converter per se deteriorates, the conversion efficiency of harmful components falls even if the air/fuel ratio is accurately controlled.
In order to solve this problem, it is necessary to determine the deterioration condition of the catalyst. However, no diagnosing system which is capable of diagnosing the performance deterioration during operation of an engine and taking fast countermeasure has been established.
Such a technology for determining the deterioration of the catalyst includes, for example, an apparatus for determining the deterioration of a catalyst for an internal combustion engine as is disclosed in Japanese Unexamined Patent Publication Tokkai-Hei 2-3091. In this apparatus, oxygen sensors are provided in the front and rear of a catalytic converter for measuring a time difference between the time when the output of the front oxygen sensor is inverted and the time when the output of the rear sensor is inverted. Deteriorated condition of the catalyst is determined based upon the measured time difference. Specifically, it is determined that the catalyst has deteriorated if the time difference is short.